Lithographic printing plate precursor

ABSTRACT

A lithographic printing plate precursor is disclosed including a support and a coating comprising a sensitizer and an onium salt wherein the onium salt and the sensitizer are capable of inducing a print-out image upon UV light exposure.

BACKGROUND ART

Lithographic printing typically involves the use of a so-called printing master such as a printing plate which is mounted on a cylinder of a rotary printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-adhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.

Lithographic printing masters are generally obtained by the image-wise exposure and processing of a radiation sensitive layer on a lithographic support. Imaging and processing renders the so-called lithographic printing plate precursor into a printing plate or master. Image-wise exposure of the radiation sensitive coating to heat or light, typically by means of a digitally modulated exposure device such as a laser, triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by cross-linking of a polymer or by particle coagulation of a thermoplastic polymer latex, solubilization by the destruction of intermolecular interactions or by increasing the penetrability of a development barrier layer. Although some plate precursors are capable of producing a lithographic image immediately after exposure, the most popular lithographic plate precursors require wet processing since the exposure produces a difference in solubility or difference in rate of dissolution in a developer between the exposed and the non-exposed areas of the coating. In positive working lithographic plate precursors, the exposed areas of the coating dissolve in the developer while the non-exposed areas remain resistant to the developer. In negative working lithographic plate precursors, the non-exposed areas of the coating dissolve in the developer while the exposed areas remain resistant to the developer. Most lithographic plate precursors contain a hydrophobic coating on a hydrophilic support, so that the areas which remain resistant to the developer define the ink-accepting, hence printing areas of the plate while the hydrophilic support is revealed by the dissolution of the coating in the developer at the non-printing areas.

Photopolymer printing plates rely on a working-mechanism whereby the coating—which typically includes free radically polymerisable compounds—hardens upon exposure. “Hardens” means that the coating becomes insoluble or non-dispersible in the developing solution and may be achieved through polymerization and/or crosslinking of the photosensitive coating upon exposure to light. Photopolymer plate precursors can be sensitized to blue, green or red light i.e. wavelengths ranging between 450 and 750 nm, to violet light i.e. wavelengths ranging between 350 and 450 nm or to infrared light i.e. wavelengths ranging between 750 and 1500 nm. Optionally, the exposure step is followed by a heating step to enhance or to speed-up the polymerization and/or crosslinking reaction.

In general, a toplayer or protective overcoat layer over the imageable layer is required to act as an oxygen barrier to provide the desired sensitivity to the plate. A toplayer typically includes water-soluble or water-swellable polymers such as for example polyvinylalcohol. Besides acting as barrier for oxygen, the toplayer should best be easily removable during processing and be sufficiently transparent for actinic radiation, e.g. from 300 to 450 nm or from 450 to 750 nm or from 750 to 1500 nm.

The classical workflow of photopolymer plates involves first an exposure step of the photopolymer printing plate precursor in a violet or infrared platesetter, followed by an optional pre-heat step, a wash step of the protective overcoat layer, an alkaline developing step, and a rinse and gum step. Over the past years, there is a clear evolution in the direction of a simplified workflow where the pre-heat step and/or wash step are eliminated and where the processing and gumming step are carried out in one single step or where processing is carried out with a neutral gum and then gummed in a second step. Alternatively, on-press processing wherein the plate is mounted on the press and the coating layer is developed by interaction with the fountain and ink that are supplied to the plate during the press run, has become very popular. During the first runs of the press, the non-image areas are removed from the support and thereby define the non-printing areas of the plate.

Besides allowing for the evaluation of the image quality, a high contrast between the image and the hydrophilic support is required in order to obtain a good image registration (alignment) of the different printing plates in multi-colour printing in order to ensure image sharpness (resolution) and a correct rendering of the colours in the images present. In order to be able to evaluate the lithographic printing plates for image quality, such as for example image resolution and detail rendering (usually measured with an optical densitometer) before mounting them on the press, the lithographic printing plate precursors often contain a colorant such as a dye or a pigment in the coating. Such colorants provide, after processing, a contrast between the image areas containing the colorant and the hydrophilic support where the coating has been removed which enables the end-user to evaluate the image quality and/or to establish whether or not the precursor has been exposed to light.

However, for photopolymer lithographic printing plates which are processed on-press and thus development of the plate is not carried out before mounting the plate on the press, a previous inspection and discrimination of the plate including colorants is not possible. A solution has been provided in the art by including components to the coating which are able to form upon exposure a so-called “print-out image”, i.e. an image which is visible before processing. In these materials however, often the photo-initiating system is a reacting component, which induces formation of the print-out image upon exposure, and therefore the lithographic differentiation may be reduced.

Contrast-providing colorants obtained from the so-called leuco dyes that switch colour upon changes in pH, temperature, UV etc, have been widely used in the art. The leuco dye technology involves a switch between two chemical forms whereby one is colourless. If the colour switch is caused by for example pH or temperature, the transformation is reversible. Irreversible switches are based on redox reactions.

Formation of a print-out image for violet sensitized photopolymer systems have been disclosed in for example U.S. Pat. Nos. 3,359,109; 3,042,515; 4,258,123; 4,139,390; 5,141,839; 5,141,842; 4,336,319; 4,232,106; 4,425,424; 5,030,548; 4,598,036; EP 434 968; WO 96/35143.

Despite the solutions which have been provided in the prior art, there is still an interest in photopolymer plate coating formulations which offer an improved contrast between the image areas and background areas and which are preferably designed for direct on-press development.

SUMMARY OF INVENTION

It is therefore an object of the present invention to provide a negative-working, violet sensitive printing plate based on photopolymerisation and/or crosslinking which offers an excellent visual contrast upon imaging, even before processing.

This object is realised by the printing plate precursor defined in claim 1 with preferred embodiments defined in the dependent claims. The invention has the specific feature that the printing plate material includes a coating comprising an onium compound having an anion as counter ion and at least one sensitizer which are capable of forming a printout image upon UV exposure.

It has surprisingly been observed that upon UV light exposure of the coating according to the present invention, a print-out image is formed without the presence of any additional components such as for example a colorant.

It is a further object of the present invention to provide a method for making a lithographic printing plate comprising the steps of:

-   -   image-wise exposing the printing plate precursor including the         coating as defined above to UV radiation whereby a lithographic         image consisting of image areas and non-image areas is formed         and whereby a colour change in the image areas is induced;     -   developing the exposed precursor.

The CIE 1976 colour distance ΔE measured before development and after exposure with UV light having an energy density between 10 and 150 mJ/cm², more preferably between 15 and 120 mJ/cm², most preferably of maximum between 20 and 100 mJ/cm², between the exposed and non-exposed areas preferably has a value of at least 5.

The development is preferably carried out by treating the precursor with a gum solution, however more preferably by mounting the precursor on a plate cylinder of a lithographic printing press and rotating the plate cylinder while feeding dampening liquid and/or ink to the precursor.

Other features, elements, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention. Specific embodiments of the invention are also defined in the dependent claims.

DESCRIPTION OF EMBODIMENTS

The Sensitizer

The lithographic printing plate precursor of the current invention comprises a coating including at least one sensitizer. Preferred sensitizers are blue, light absorbing sensitizers having an absorption spectrum between 320 nm and 500 nm. Preferably, the sensitizer has a structure according to the following Formula's I or II:

wherein R^(1′) to R^(5′) and R¹″ to R⁵″ independently represent hydrogen, an alkyl group, an alkoxy group, a cyano group or a halogen;

wherein R¹ to R¹⁴ independently represent hydrogen, an alkyl group, an alkoxy group, a cyano group or a halogen.

One of R^(1′) to R^(5′) or R¹″ to R⁵″ in Formula I preferably represents an alkoxy group having more than 1 carbon atom.

More preferably, R^(1′), R^(5′), R¹″, R⁵″ in Formula I independently represent hydrogen, fluorine, chlorine, R^(2′) to R^(4′) and R²″ to R⁴″ in Formula I independently represent an alkoxy group; and at least two of the alkoxy groups are branched and have from 3 to 15 carbon atoms

Even more preferred, R^(1′), R^(5′), R¹″, R⁵ in Formula I represent hydrogen and R^(2′) to R^(4′) and R²″ to R⁴″ in Formula I independently represent an alkoxy group; and at least two of the alkoxy groups are branched and have from 3 to 15 carbon atoms.

Most preferred, R^(2′), R^(4′), R²″, R⁴″ in Formula I represent a methoxy group and R^(3′) and R³″ in Formula I independently represent branched alkoxy groups having 3 to 15 carbon atoms.

One of R¹ to R¹⁰ in Formula II preferably represents an alkoxy group having more than 1 carbon atom.

More preferably, R¹, R⁵, R⁶, R¹⁰, R¹¹, R¹², R¹³ and R¹⁴ in Formula II independently represent hydrogen, fluorine, chlorine, R² to R⁴ and R⁷ to R⁹ in Formula II independently represent an alkoxy group; and at least two of the alkoxy groups are branched and have from 3 to 15 carbon atoms

Even more preferred, R¹, R⁵, R⁶, and R¹⁰ in Formula II represent hydrogen and R² to R⁴, and R⁷ to R⁹ in Formula II independently represent an alkoxy group; and at least two of the alkoxy groups are branched and have from 3 to 15 carbon atoms.

Most preferred, R², R⁴, R⁷ and R⁹ in Formula II represent a methoxy group and R³ and R⁸ in Formula II independently represent branched alkoxy groups having 3 to 15 carbon atoms.

The sensitizers can be used as single compound or as mixture of several compounds. The overall amount of these compounds is preferably comprised between 0.1 to 25% by weight, more preferably between 0.5 to 20% by weight by weight and most preferably between 1.0 to 15% by weight with respect to the total weight of the non-volatile compounds in the composition. The print-out image is already obtained at a low concentration of sensitizer; for example at an amount of 0.1% wt to 6% wt.

The following structures are examples of preferred sensitizers used in the present invention.

The coating of the printing plate precursor of the present invention most preferably contains at least one of the following compounds:

The three sensitizers according to Formulae III to V may be used in the coating as a mixture. The mixture preferably contains between 30 and 45% by weight of Formula (III); between 35 and 55% by weight of Formula (IV) and between 5 and 25% by weight of Formula (V).

The Onium Compound

The coating of the printing plate precursor includes at least one onium salt including an anion as counter ion. Preferred onium salts include iodonium salts and sulfonium salts. One or a mixture of two or more onium salts may be present in the coating.

Preferred examples of iodonium salts without being limited thereto include optionally substituted diaryl iodonium salts or diheteroaryl iodonium salts. Diaryl iodonium salts or diheteroaryl iodonium salts substituted with Specific examples of the diaryliodonium salts include diphenyliodonium, 4-methoxyphenyl-4-(2-methylpropyl) phenyliodonium, 4-chlorophenyl-4-phenyliodonium, 4-(2-methylpropyl) phenyl-tolyl iodonium, 4-hexyloxyphenyl-2,4,6-trimethoxyphenyliodonium, 4-hexyloxyphenyl-2,4-diethoxyphenyliodonium, 4-octyloxyphenyl-6-trimethoxyphenyliodonium, bis (4-tert-butylphenyl) iodonium and bis (4-isopropylphenyl) iodonium, 4-octyloxyphenyl phenyliodonium, [4-[(2-hydroxytetradecyl)-oxy]phenyl]phenyliodonium, 4-methylphenyl-4′-hexylphenyliodonium tetraphenylborate, 4-methylphenyl-4′-cyclohexylphenyliodonium, 4-hexylphenyl-phenyliodonium, 4-methylphenyl-4′cyclohexylphenyliodonium, 4-cyclohexylphenyl-phenyliodonium, 2-methyl-4-t-butylphenyl4′-methylphenyliodonium. Mixtures of two or more of these onium salts can be selected.

Preferred examples of the triarylsulfonium salts without being limited thereto include triphenylsulfonium, dialkylphenacylsulfonium, dialkyl-4-hydroxyphenylsulfonium, bis (4-chlorophenyl) phenylsulfonium, triphenylsulfonium benzoyl formate, bis (4-chlorophenyl) phenylsulfonium benzoyl formate, bis (4-chlorophenyl)-4-methylphenylsulfonium bis (4-chlorophenyl)-4-methylphenylsulfonium, tris (4-chlorophenyl) sulfonium, tris 2,4-dichlorophenyl) sulfonium, bis (2,4-dichlorophenyl) phenyl sulfonium and bis (2,4-dichlorophenyl) 4-methoxyphenyl sulfonium. Mixtures of two or more of these onium salts can be selected.

According to the present invention, the counter ion of the onium salts is selected from hexafluorophosphate (PF₆ ⁻), SbF₆ ⁻ or AsF₆ ⁻.

The onium salts are preferably present in the coating in an amount between at least 1% wt and 25% wt, more preferably in an amount between at least 5% wt and 20% wt, most preferably in an amount between at least 10% wt and 15% wt, all based on the total dry weight of the photopolymerisable and/or crosslinkable layer.

The colour difference between the exposed and non-exposed areas of the coating calculated from the L*a*b* values of the exposed areas of the image areas (exposed areas) of the coating and the L*a*b* values of non-image areas (non-exposed areas) of the coating, is denoted as ΔE. It has surprisingly been found that upon exposure of the coating of the present invention with UV radiation, a print-out image is formed characterised by a CIE 1976 colour difference ΔE of at least 5, more preferably at least 6 and most preferably at least 10. ΔE is the CIE 1976 colour distance Delta E that is defined by the pair wise Euclidean distance of the CIE L*a*b* colour coordinates. CIE L*a*b* colour coordinates are obtained from reflection measurement in 45/0 geometry (non-polarized), using CIE 2° observer and D50 as illuminant. More details are described in CIE S 014-4/E: 2007 Colourimetry—Part 4: CIE 1976 L*a*b* Colour Spaces and CIE publications and CIE S 014-1/E:2006, CIE Standard Colourimetric Observers.

The CIE 1976 colour coordinates L*, a* and b* discussed herein are part of the well-known CIE (Commission Internationale de l'Eclairage) system of tristimulus colour coordinates, which also includes the additional chroma value C* defined as C*=[(a)²+(b)²]^(1/2). The CIE 1976 colour system is described in e.g. “Colorimetry, CIE 116-1995: Industrial Colour Difference Evaluation”, or in “Measuring Colour” by R. W. G. Hunt, second edition, edited in 1992 by Ellis Horwood Limited, England.

CIE L*a*b* values discussed and reported herein have been measured following the ASTM E308-85 method.

At the exposed areas of the coating the L*, a* and b* coordinates are enhanced due to absorption in the visual wavelength range whereby a clear print-out image is formed. The print-out image is visible due to the contrast of the image which is defined as the colour difference between the exposed areas and the non-exposed areas. This contrast is preferably as high as possible and enables the end-user to establish immediately after imaging whether or not the precursor has already been exposed, to distinguish the different colour selections and to inspect the quality of the image on the plate precursor.

Definitions

The optionally substituted aryl herein is preferably an optionally substituted phenyl, benzyl, tolyl or an ortho-meta- or para-xylyl, naphtyl, anthracenyl, phenanthrenyl, and/or combinations thereof. The heteroaryl group is preferably a monocyclic or polycyclic aromatic ring comprising carbon atoms and one or more heteroatoms in the ring structure, preferably, 1 to 4 heteroatoms, independently selected from nitrogen, oxygen, selenium and sulphur. Preferred examples thereof include an optionally substituted furyl, pyridinyl, pyrimidyl, pyrazoyl, imidazoyl, oxazoyl, isoxazoyl, thienyl, tetrazoyl, thiazoyl, (1,2,3)triazoyl, (1,2,4)triazoyl, thiadiazoyl, thiofenyl group and/or combinations thereof and the optionally substituted heteroaryl is preferably a five- or six-membered ring substituted by one, two or three oxygen atoms, nitrogen atoms, sulphur atoms, selenium atoms or combinations thereof. Examples thereof include furan, thiophene, pyrrole, pyrazole, imidazole, 1,2,3-triazole, 1,2,4-triazole, tetrazole, oxazole, isoxazole, thiazole, isothiazole, thiadiazole, oxadiazole, pyridine, pyridazine, pyrimidine, pyrazine, 1,3,5-triazine, 1,2,4-triazine or 1,2,3-triazine, benzofuran, benzothiophene, indole, indazole, benzoxazole, quinoline, quinazoline, benzimidazole or benztriazole.

The term “alkyl” herein means all variants possible for each number of carbon atoms in the alkyl group i.e. methyl, ethyl, for three carbon atoms: n-propyl and isopropyl; for four carbon atoms: n-butyl, isobutyl and tertiary-butyl; for five carbon atoms: n-pentyl, 1,1-dimethyl-propyl, 2,2-dimethylpropyl and 2-methyl-butyl, etc. Preferably, the alkyl group is a C₁ to C₂₀-alkyl group; more preferably the alkyl group is a C₁ to C₆-alkyl group. Most preferably the alkyl is a methyl group. Cycloalkyls include for example, substituted or unsubstituted cyclopropyl, cyclopentyl, cyclohexyl, 4-methylcyclohexyl, and cyclooctyl groups.

The term “substituted”, in e.g. substituted alkyl group means that the alkyl group may be substituted by other atoms than the atoms normally present in such a group, i.e. carbon and hydrogen. For example, a substituted alkyl group may include a halogen atom or a thiol group. An unsubstituted alkyl group contains only carbon and hydrogen atoms.

The optional substituents represent an alkyl, cycloalkyl, alkenyl or cyclo alkenyl group, an alkynyl group, an aryl or heteroaryl group, an alkylaryl or arylalkyl group, an alkoxy group such as methoxy, ethoxy, iso-propoxy, t-butoxy, (2-hydroxytetradecyl)oxy, and various other linear and branched alkyleneoxyalkoxy groups; an aryloxy group, a thio alkyl, thio aryl or thio heteroaryl group, a hydroxyl group, —SH, a carboxylic acid group or an alkyl ester thereof, a sulphonic acid group or an alkyl ester thereof, a phosphonic acid group or an alkyl ester thereof, a phosphoric acid group or an alkyl ester thereof, an amino group, a sulphonamide group, an amide group, a nitro group, a nitrile group, a halogen such as fluoro, chloro, or bromo, or a combination thereof.

A suitable alkenyl group herein is preferably a C₂ to C₆-alkenyl group such as an ethenyl, n-propenyl, n-butenyl, n-pentenyl, n-hexenyl, iso-propenyl, iso-butenyl, iso-pentenyl, neo-pentenyl, 1-methylbutenyl, iso-hexenyl, cyclopentenyl, cyclohexenyl and methylcyclohexenyl group.

A suitable alkynyl group herein is preferably a C₂ to C₆-alkynyl group; a suitable aralkyl group is preferably a phenyl group or naphthyl group including one, two, three or more C₁ to C₆-alkyl groups; a suitable alkaryl group is preferably a C₁ to C₆-alkyl group including an aryl group, preferably a phenyl group or naphthyl group.

A cyclic group or cyclic structure herein includes at least one ring structure and may be a monocyclic- or polycyclic group, meaning one or more rings fused together.

The Lithographic Printing Plate Precursor

The lithographic printing plate precursor according to the present invention is negative-working, i.e. after exposure and development the non-exposed areas of the coating are removed from the support and define hydrophilic (non-printing) areas, whereas the exposed coating is not removed from the support and defines oleophilic (printing) areas. The hydrophilic areas are defined by the support which has a hydrophilic surface or is provided with a hydrophilic layer. The hydrophobic areas are defined by the coating, hardened upon exposing, optionally followed by a heating step. Areas having hydrophilic properties means areas having a higher affinity for an aqueous solution than for an oleophilic ink; areas having hydrophobic properties means areas having a higher affinity for an oleophilic ink than for an aqueous solution.

“Hardened” means that the coating becomes insoluble or non-dispersible for the developing solution and may be achieved through polymerization and/or crosslinking of the photosensitive coating, optionally followed by a heating step to enhance or to speed-up the polymerization and/or crosslinking reaction. In this optional heating step, hereinafter also referred to as “pre-heat”, the plate precursor is heated, preferably at a temperature of about 80° C. to 150° C. and preferably during a dwell time of about 5 seconds to 1 minute.

The coating includes at least one layer including a photopolymerisable and/or crosslinkable composition. The layer including the mainly photopolymerisable composition is also referred to as the “photopolymerisable layer”, the layer including the mainly crosslinkable composition is also referred to as the “crosslinkable layer”. The coating may include an intermediate layer, located between the support and the photopolymerisable and/or crosslinkable layer. The lithographic printing precursors can be multi-layer imageable elements.

The lithographic printing plate precursor can be prepared by applying on a support the coating as described below and drying the precursor.

The printing plate of the present invention is preferably exposed with UV radiation having an energy density comprised between 10 mJ/cm² and 150 mJ/cm²; preferably between 15 mJ/cm² and 120 mJ/cm²; most preferably between 20 mJ/cm² and 100 mJ/cm². The printing plate precursor of the present invention has the specific feature that it is not sensitive to the portion of the electromagnetic spectrum starting from about 500 nm and higher. In other words, the printing plate of the present invention is stable for at least five minutes in office light (i.e. for example 800 lux). Stable means that the quality of the printing plate precursor remains high and that the precursor does not have an increased tendency to toning and/or formation of defects in the coating after such light exposure. Also, the sensitivity of the plate precursor does not reduce after such light exposure. Thus the quality of the printing plate remains high i.e. no/limited loss of sensitivity, no/limited formation of defects in the coating, and/or no/limited tendency of toning after exposure to office light up to five minutes.

Support

The lithographic printing plate used in the present invention comprises a support which has a hydrophilic surface or which is provided with a hydrophilic layer. The support is preferably a grained and anodized aluminum support, well known in the art. Suitable supports are for example disclosed in EP 1 843 203 (paragraphs [0066] to [0075]). The surface roughness, obtained after the graining step, is often expressed as arithmetical mean center-line roughness Ra (ISO 4287/1 or DIN 4762) and may vary between 0.05 and 1.5 μm. The aluminum substrate of the current invention has preferably an Ra value below 0.45 μm, more preferably below 0.40 μm and most preferably below 0.30 μm. The lower limit of the Ra value is preferably about 0.1 μm. More details concerning the preferred Ra values of the surface of the grained and anodized aluminum support are described in EP 1 356 926. By anodising the aluminum support, an Al₂O₃ layer is formed and the anodic weight (g/m² Al₂O₃ formed on the aluminum surface) varies between 1 and 8 g/m². The anodic weight is preferably ≥3 g/m², more preferably ≥3.5 g/m² and most preferably ≥4.0 g/m²

The grained and anodized aluminum support may be subjected to so-called post-anodic treatments, for example a treatment with polyvinylphosphonic acid or derivatives thereof, a treatment with polyacrylic acid, a treatment with potassium fluorozirconate or a phosphate, a treatment with an alkali metal silicate, or combinations thereof. Alternatively, the support may be treated with an adhesion promoting compound such as those described in EP 1 788 434 in [0010] and in WO 2013/182328. However, for a precursor optimized to be used without a pre-heat step it is preferred to use a grained and anodized aluminum support without any post-anodic treatment.

Besides an aluminum support, a plastic support, for example a polyester support, provided with one or more hydrophilic layers as disclosed in for example EP 1 025 992 may also be used.

The Photopolymerisable Layer

The photopolymerisable layer includes, besides the sensitizer and the onium salt discussed above, a polymerisable compound, optionally a binder and a polymerization initiator capable of hardening said polymerisable compound in the exposed areas. The photopolymerisable layer has a coating thickness preferably ranging between 0.2 and 5.0 g/m², more preferably between 0.4 and 3.0 g/m², most preferably between 0.6 and 2.2 g/m².

Polymerisable Compound and Initiator

The polymerisable compound is preferably a monomer or oligomer including at least one epoxy or vinyl ether functional group and the polymerisation initiator is a Brönsted acid generator capable of generating free acid, optionally in the presence of a sensitizer, upon exposure, hereinafter the Brönsted acid generator is also referred to as “cationic photoinitiator” or “cationic initiator”.

Suitable polyfunctional epoxy monomers include, for example, 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate, bis-(3,4-epoxycyclohexymethyl) adipate, difunctional bisphenol A-epichlorohydrin epoxy resin and multifunctional epichlorohydrintetraphenylol ethane epoxy resin.

Suitable cationic photoinitiators include, for example, triarylsulfonium hexafluoroantimonate, triarylsulfonium hexafluorophosphate, diaryliodonium hexafluoroantimonate, and haloalkyl substituted s-triazine. It is noted that most cationic initiators are also free radical initiators because, in addition to generating Brönsted acid, they also generate free radicals during photo or thermal decomposition.

According to a more preferred embodiment of the present invention, the further polymerisable compound is a polymerisable monomer or oligomer including at least one terminal ethylenic group, hereinafter also referred to as “free-radical polymerisable monomer”, and the polymerisation initiator is a compound capable of generating free radicals upon exposure, optionally in the presence of a sensitizer, hereinafter said initiator is referred to as “free radical initiator”. The polymerisation involves the linking together of the free-radical polymerisable monomers.

Suitable free-radical polymerisable monomers include, for example, multifunctional (meth)acrylate monomers (such as (meth)acrylate esters of ethylene glycol, trimethylolpropane, pentaerythritol, ethoxylated ethylene glycol and ethoxylated trimethylolpropane, multifunctional urethanated (meth)acrylate, and epoxylated (meth)acrylate), and oligomeric amine diacrylates. The (meth)acrylic monomers may also have other double bond or epoxide group, in addition to (meth)acrylate group. The (meth)acrylate monomers may also contain an acidic (such as carboxylic acid) or basic (such as amine) functionality.

Suitable free-radical polymerisable monomers are disclosed in [0042] and [0050] of EP 2 916 171 and are incorporated herein by reference.

The coating contains a free radical initiator capable of generating free radicals upon exposure directly and/or in the presence of a sensitizer. Suitable free-radical initiators are described in WO 2005/111727 from page 15 line 17 to page 16 line 11 and EP 1 091 247 and may include for example hexaaryl-bisimidazole compound (HABI; dimer of triaryl-imidazole), aromatic ketones, aromatic onium salts, organic peroxides, thio compounds, ketooxime ester compounds, borate compounds, azinium compounds, metallocene compounds, active ester compounds and further compounds having a carbon-halogen bond.

The photopolymerisable layer may also comprise a co-initiator. Typically, a co-initiator is used in combination with a free radical initiator. Suitable co-initiators for use in the photopolymer coating are disclosed in U.S. Pat. Nos. 6,410,205; 5,049,479; EP 1 079 276, EP 1 369 232, EP 1 369 231, EP 1 341 040, US 2003/0124460, EP 1 241 002, EP 1 288 720 and in the reference book including the cited refences: Chemistry & Technology UV & EB formulation for coatings, inks & paints—Volume 3—Photoinitiators for Free Radical and Cationic Polymerisation by K. K. Dietliker—Edited by P. K. T. Oldring—1991—ISBN 0 947798161. Specific co-initiators, as described in EP 107 792, may be present in the photopolymerisable layer to further increase the sensitivity. Preferred co-initiators are disclosed in EP 2 916 171 [0051] and are incorporated herein by reference.

The Binder

The photopolymerisable layer preferably includes a binder. The binder can be selected from a wide series of organic polymers. Compositions of different binders can also be used. Useful binders are described in WO2005/111727 page 17 line 21 to page 19 line 30, EP 1 043 627 in paragraph [0013] and in WO2005/029187 page 16 line 26 to page 18 line 11.

Further Ingredients

The photopolymerisable layer may also comprise particles which increase the resistance of the coating against manual or mechanical damage. The particles may be inorganic particles, organic particles or fillers such as described in for example U.S. Pat. No. 7,108,956. More details of suitable spacer particles described in EP 2 916 171 [0053] to [0056] are incorporated herein by reference.

The photopolymerisable layer may also comprise an inhibitor. Particular inhibitors for use in the photopolymer coating are disclosed in U.S. Pat. No. 6,410,205, EP 1 288 720 and EP 1 749 240.

The photopolymerisable layer may further comprise an adhesion promoting compound. The adhesion promoting compound is a compound capable of interacting with the support, preferably a compound having an addition-polymerisable ethylenically unsaturated bond and a functional group capable of interacting with the support. Under “interacting” is understood each type of physical and/or chemical reaction or process whereby, between the functional group and the support, a bond is formed which can be a covalent bond, an ionic bond, a complex bond, a coordinate bond or a hydrogen-bond, and which can be formed by an adsorption process, a chemical reaction, an acid-base reaction, a complex-forming reaction or a reaction of a chelating group or a ligand.

The adhesion promoting compound may be selected from at least one of the low molecular weight compounds or polymeric compounds as described in EPA 851 299 from lines 22 on page 3 to line 1 on page 4, EP-A 1 500 498 from paragraph [0023] on page 7 to paragraph [0052] on page 20, EP-A 1 495 866 paragraph [0030] on page 5 to paragraph [0049] on page 11, EP-A 1 091 251 from paragraph [0014] on page 3 to paragraph [0018] on page 20, and EP-A 1 520 694 from paragraph [0023] on page 6 to paragraph [0060] on page 19. Preferred compounds are those compounds which comprise a phosphate or phosphonate group as functional group capable of adsorbing on the aluminum support and which comprise an addition-polymerisable ethylenic double bond reactive group, especially those described in EP-A 851 299 from lines 22 on page 3 to line 1 on page 4 and EP-A 1 500 498 from paragraph [0023] on page 7 to paragraph [0052] on page 20. Also preferred are those compounds which comprises a tri-alkyl-oxy silane groups, hereinafter also referred to as “trialkoxy silane” groups, wherein the alkyl is preferably methyl or ethyl, or wherein the trialkyloxy silane groups are at least partially hydrolysed to silanol groups, as functional group capable of adsorbing on the support, especially silane coupling agents having an addition-polymerisable ethylenic double bond reactive group as described in EP-A 1 557 262 paragraph [0279] on page 49 and EP-A 1 495 866 paragraph [0030] on page 5 to paragraph [0049] on page 11. Also the adhesion promoting compounds described in EP 2 916 171 [0058] are incorporated herein by reference.

The adhesion promoting compound may be present in the photopolymerisable layer in an amount ranging between 1 and 50 wt %, preferably between 3 and 30 wt %, more preferably between 5 and 20 wt % of the non-volatile components of the composition.

The adhesion promoting compound may be present in an optional intermediate layer in an amount of at least 25 wt %, preferably at least 50 wt %, more preferably at least 75 wt %, of the non-volatile components of the composition. Alternatively, the intermediate layer may consist of the adhesion promoting compound.

Various surfactants may be added into the photopolymerisable layer to allow or enhance the developability of the precursor; especially developing with a gum solution. Both polymeric and small molecule surfactants for example nonionic surfactants are preferred. More details are described in EP 2 916 171 [0059] and are incorporated herein by reference.

The Crosslinkable Layer

Based on a Diazonium Compound

The crosslinkable layer may include a diazonium compound and preferably a binder.

Diazonium compounds are preferably characterized by the generic structure A-N₂ ⁺X⁻, wherein A is an aromatic or heterocyclic residue and X is the anion of an acid. Specific examples of light sensitive diazonium coatings include higher molecular weight compositions obtained, for example, by the condensation of certain aromatic diazonium salts in an acid condensation medium with active carbonyl compounds such as formaldehyde, as disclosed for example in U.S. Pat. Nos. 2,063,631 and 2,667,415. Suitable examples include condensation products of diazonium salts of p-amino-diphenylamines, such as diphenylamine-4-diazonium chloride or diphenylamine-4-diazonium bromide or diphenyl-amine-4-diazonium phosphate, with formaldehyde in phosphoric acid of high concentration. The term phosphoric acid also includes pyrophosphoric acid, metaphosphoric acid, and poly-phosphoric acid.

Another preferred class of diazonium compounds is described in U.S. Pat. No. 3,849,392. The compounds are the polycondensation product of 3-methoxy-4-diazo-diphenyl amine sulfate and 4, 4′-bis-methoxy methyl-diphenyl ether, precipitated as mesitylene sulfonate, as taught in U.S. Pat. No. 3,849,392. The most preferred diazonium salt is benzenediazonium, 2-methoxy-4-(phenylamino), 2,4,6-trimethylbenzenesulfonate (1:1), polymer with 1,1′-oxybis[4-(methoxymethyl)benzene]. The preparation of this diazonium salt is disclosed in DE 2024244A. Other diazonium salts disclosed in this document are suitable to be contained in the crosslinkable layer.

The diazonium salt is preferably present in the coating composition in an amount of from about 20% to about 100% by weight of the solid composition components. A more preferred range is from about 25% to 50% and most preferably from about 30% to 45%.

The binder may be added to the diazonium compound to improve mechanical resistance of the crosslinkable layer and/or the processing behaviour of the plate.

Suitable binders are polyvinyl acetates, epoxy resins based on bis-phenol-A-epichlorohydrin, p-(vinyl butyral-co-,vinyl acetate-co-vinyl alcohol), unplasticized urea resin of an approximate acid number of 2 (Resamin 106 F), Recinene-modified alkyd resin, Resins comprising a polyvinyl acetate resin and a styrene/maleic acid half ester copolymer,

Suitable polyvinyl acetate resins have a weight average molecular weight in the range of from about 40.000 to less than 800.000. A preferred weight average molecular weight maximum is about 700.000; more preferably 680.000. The most preferred average molecular weight is in the range of about 80.000 to 200,000. Preferred binders are the butyl semi-ester of the maleic acid anhydride/styrene copolymers (such as Scripset® 540, available from Monsanto) and the styrene/maleic acid half ester copolymers as disclosed in U.S. Pat. No. 4,511,640A. A more preferred binder is obtained by reacting p-[vinylbutyral-co-vinyl alcohol-co-vinyl acetate] such as Mowital B30T or Mowital B60T (from Kuraray Europe GmbH) with maleic acid anhydride to a half-ester and half acid, with the OH of the polyvinylalcohol as disclosed in Preparation Example 5 in U.S. Pat. No. 5,695,905.

The binder is preferably present in the coating composition in an amount of from about 8% to about 60% by weight of the solid composition components. A more preferred range is from about 12% to 50% and most preferably from about 18% to 45%.

The weight ratio of binder to diazonium compound does not exceed 20, preferably equal to or less than 10, more preferably between 0.8 and 1.2.

The coverage of the crosslinkable layer is preferably between 0.1 and 1.2 g/m², more preferably between 0.5 and 0.8 g/m².

The crosslinkable layer may further comprise additives, such as for example acid stabilizers including phosphoric, citric, tartaric and p-toluene sulfonic acids. The acid stabilizer may be present in the coating composition in an amount of from about 1.5% to about 4.5% by weight of the solid composition components, a more preferably from about 2.0% to 4.0% and most preferably from about 2.5% to 3.5%.

Exposure indicators including para phenyl azo diphenyl amine, Calcozine Fuchine dyes and Crystal Violet and Methylene Blue dyes may be present in an amount from about 0.05% to about 0.35% by weight of the solid composition components. A more preferred range is from about 0.10% to 0.30% and most preferably from about 0.15% to 0.25%.

Suitable solvents which may be used as a medium to combine the ingredients of the coating include Methyl Cellosolve, ethylene glycol ethers, butyrolactone, alcohols as ethyl alcohol and n-propanol, and ketones such as methyl ethyl ketone.

Based on o-quinonediazide

Alternatively, the crosslinkable layer may include an o-quinonediazide compound and preferably a binder.

o-quinonediazide compounds are well-known and are for example described in Light-Sensitive Systems written by J. Kosar (Published by John Wiley & Sons. Inc.) pp. 339-352. Suitable o-quinonediazide compounds are for example o-naphthoquinonediazide sulfonic acid esters of aromatic hydroxyl compounds, o-naphthoquinonediazide carboxylic acid esters of aromatic hydroxy compounds, o naphthoquinonediazide sulfonic acid amides of aromatic amino compounds and o-naphthoquinonediazide carboxylic acid amides of aromatic amino compounds.

Preferred binders are alkali-soluble resins include novolak resins such as for examples phenol-formaldehyde resins, cresol-formaldehyde resins, p-tert-butylphenol-formaldehyde resins, phenol modified xylene resins, i.e., a formaldehyde condensate of phenol and xylene, and phenol modified xylene mesitylene resins, i.e., a formaldehyde condensate of phenol, xylene and mesitylene. Other useful alkali-soluble resins include polyhydroxystyrene, polyhalogenated hydroxystyrene, and copolymers of acrylic acid or methacrylic acid and other vinyl compounds (for example, methyl methacrylate).

The amount of o-quinonediazide compounds in coating composition is preferably from 10 to 50% by weight, preferably from 20 to 40% by weight, based on the total solid composition components. The amount of the alkali-soluble resins added is preferably between 90 and 50% by weight, preferably between 80 and 60% by weight, based on the total solid weight.

The coating may include on the photopolymerisable or on the crosslinkable layer, a toplayer or protective overcoat layer which acts as an oxygen barrier layer including water-soluble or water-swellable binders. Printing plate precursors which do not contain a toplayer or protective overcoat layer are also referred to as overcoat-free printing plate precursors. In the art, it is well-known that low molecular weight substances present in the air may deteriorate or even inhibit image formation and therefore usually a toplayer is applied to the coating. A toplayer should be easily removable during development, adhere sufficiently to the photopolymerisable layer or optional other layers of the coating and should preferably not inhibit the transmission of light during exposure. Preferred binders which can be used in the toplayer are polyvinyl alcohol and the polymers disclosed in WO 2005/029190; U.S. Pat. No. 6,410,205 and EP 1 288 720, including the cited references in these patents and patent applications. The most preferred binder for the toplayer is polyvinylalcohol. The polyvinylalcohol has preferably a hydrolysis degree ranging between 74 mol % and 99 mol %, more preferably between 88-98%. The weight average molecular weight of the polyvinylalcohol can be measured by the viscosity of an aqueous solution, 4% by weight, at 20° C. as defined in DIN 53 015, and this viscosity number ranges preferably between 1 and 26, more preferably between 2 and 15, most preferably between 2 and 10.

The overcoat layer may optionally include other ingredients such as inorganic or organic acids, matting agents or wetting agents as disclosed in EP 2 916 171 and are incorporated herein by reference.

The coating thickness of the optional toplayer is preferably between 0.25 and 1.75 g/m², more preferably between 0.25 and 1.3 g/m², most preferably between 0.25 and 1.0 g/m². In a more preferred embodiment of the present invention, the optional toplayer has a coating thickness between 0.25 and 1.75 g/m² and comprises a polyvinylalcohol having a hydrolysis degree ranging between 74 mol % and 99 mol % and a viscosity number as defined above ranging between 1 and 26.

In a preferred embodiment of the present invention, the photopolymerisable or the crosslinkable layer does not contain an overcoat layer.

According to the present invention there is also provided a method for making a negative-working lithographic printing plate comprising the steps of imagewise exposing a printing plate precursor followed by developing the imagewise exposed precursor so that the non-exposed areas are dissolved in the developer solution. Optionally, after the imaging step, a heating step is carried out to enhance or to speed-up the polymerization and/or crosslinking reaction. The lithographic printing plate precursor can be prepared by (i) applying on a support the coating as described above and (ii) drying the precursor.

Exposure Step

Preferably, the image-wise exposing step is carried out off-press in a platesetter, i.e. an exposure apparatus suitable for image-wise exposing the precursor with a laser such as a laser diode, for example emitting around 405 nm, or by a conventional exposure in contact with a mask. In a preferred embodiment of the present invention, the precursor is image-wise exposed by a laser emitting UV-light.

The printing plate precursor is preferably image wise exposed with UV-light and/or light in the short wavelength region of the visible light spectrum. The light source preferably emits light having a wavelength between 360 and 420 nm. The light source is built in an exposure head. Different modes can be used to expose the lithographic printing plate precursor, one where the plate is immobile and the exposure head moves back and forth such is in image setters having an internal drum, one where the plate is mounted onto a drum which rotates at high speed while the exposure head moves from one side of the drum to the other side of the drum.

The light source can be a bulb or lamp such as mercury vapour bulbs. Preferably lasers are used due to the high energy density achievable, such as fibre-coupled laser diodes emitting at 405 nm. This high energy density makes it possible to achieve high plate throughput during the exposure step. The image wise exposure via a laser is done by digital modulation of the current and or voltage.

The image wise exposure may be achieved by means of modulation of the light emitted from the light source. This modulation can be done by means of digital mirror devices, also called DMD imaging. UV setters which are suitable to digitally modulate the light source are available from Lüscher AG and Basysprint from Xeikon International B.V. The digital information is obtained from a digital image which is made available to the UV setter.

Preheat Step

After the exposing step, the precursor may be pre-heated in a preheating unit, preferably at a temperature of about 80° C. to 150° C. and preferably during a dwell time of about 5 seconds to 1 minute. This preheating unit may comprise a heating element, preferably an IR-lamp, an UV-lamp, heated air or a heated roll. Such a preheat step can be used for printing plate precursors comprising a photopolymerisable composition to enhance or to speed-up the polymerization and/or crosslinking reaction.

Development Step

Subsequently to the exposing step or the preheat step, when a preheat step is present, the plate precursor may be processed (developed). Before developing the imaged precursor, a pre-rinse step might be carried out especially for the negative-working lithographic printing precursors having a protective oxygen barrier or topcoat. This pre-rinse step can be carried out in a stand-alone apparatus or by manually rinsing the imaged precursor with water or the pre-rinse step can be carried out in a washing unit that is integrated in a processor used for developing the imaged precursor. The washing liquid is preferably water, more preferably tap water. More details concerning the wash step are described in EP 1 788 434 in [0026].

During the development step, the non-exposed areas of the image-recording layer are at least partially removed without essentially removing the exposed areas. The processing liquid, also referred to as developer, can be applied to the plate e.g. by rubbing with an impregnated pad, by dipping, immersing, coating, spincoating, spraying, pouring-on, either by hand or in an automatic processing apparatus. The treatment with a processing liquid may be combined with mechanical rubbing, e.g. by a rotating brush. During the development step, any water-soluble protective layer present is preferably also removed. The development is preferably carried out at temperatures between 20 and 40° C. in automated processing units.

Processing Liquid

The processing liquid may be an alkaline developer or solvent-based developer. Suitable alkaline developers have been described in US2005/0162505. An alkaline developer is an aqueous solution which has a pH of at least 11, more typically at least 12, preferably from 12 to 14. Alkaline developers typically contain alkaline agents to obtain high pH values can be inorganic or organic alkaline agents. The developers can comprise anionic, non-ionic and amphoteric surfactants (up to 3% on the total composition weight); biocides (antimicrobial and/or antifungal agents), antifoaming agents or chelating agents (such as alkali gluconates), and thickening agents (water soluble or water dispersible polyhydroxy compounds such as glycerine or polyethylene glycol).

Preferably, the processing liquid is a gum solution whereby during the development step the non-exposed areas of the photopolymerisable layer are removed from the support and the plate is gummed in a single step. The development with a gum solution has the additional benefit that, due to the remaining gum on the plate in the non-exposed areas, an additional gumming step is not required to protect the surface of the support in the non-printing areas. As a result, the precursor is processed and gummed in one single step which involves a less complex developing apparatus than a developing apparatus comprising a developer tank, a rinsing section and a gumming section. The gumming section may comprise at least one gumming unit or may comprise two or more gumming units. These gumming units may have the configuration of a cascade system, i.e. the gum solution, used in the second gumming unit and present in the second tank, overflows from the second tank to the first tank when gum replenishing solution is added in the second gumming unit or when the gum solution in the second gumming unit is used once-only, i.e. only starting gum solution is used to develop the precursor in this second gumming unit by preferably a spraying or jetting technique. More details concerning such gum development is described in EP1 788 444.

A gum solution is typically an aqueous liquid which comprises one or more surface protective compounds that are capable of protecting the lithographic image of a printing plate against contamination, e.g. by oxidation, fingerprints, fats, oils or dust, or damaging, e.g. by scratches during handling of the plate. Suitable examples of such surface protective compounds are film-forming hydrophilic polymers or surfactants. The layer that remains on the plate after treatment with the gum solution preferably comprises between 0.005 and 20 g/m² of the surface protective compound, more preferably between 0.010 and 10 g/m², most preferably between 0.020 and 5 g/m². More details concerning the surface protective compounds in the gum solution can be found in WO 2007/057348 page 9 line 3 to page 11 line 6. As the developed plate precursor is developed and gummed in one step, there is no need to post-treat the processed plate.

The gum solution preferably has a pH value between 3 and 11, more preferably between 4 and 10, even more preferably between 5 and 9, and most preferably between 6 and 8. A suitable gum solution is described in for example EP 1 342 568 in [0008] to [0022] and WO2005/111727. The gum solution may further comprise an inorganic salt, an anionic surfactant, a wetting agent, a chelate compound, an antiseptic compound, an anti-foaming compound and/or an ink receptivity agent and/or combinations thereof. More details about these additional ingredients are described in WO 2007/057348 page 11 line 22 to page 14 line 19.

Drying and Baking Step

After the processing step the plate may be dried in a drying unit. In a preferred embodiment the plate is dried by heating the plate in the drying unit which may contain at least one heating element selected from an IR-lamp, an UV-lamp, a heated metal roller or heated air.

After drying the plate can optionally be heated in a baking unit. More details concerning the heating in a baking unit can be found in WO 2007/057348 page 44 line 26 to page 45 line 20.

The printing plate thus obtained can be used for conventional, so-called wet offset printing, in which ink and an aqueous dampening liquid is supplied to the plate. Another suitable printing method uses a so-called single-fluid ink without a dampening liquid. Suitable single-fluid inks have been described in U.S. Pat. Nos. 4,045,232; 4,981,517 and 6,140,392. In a most preferred embodiment, the single-fluid ink comprises an ink phase, also called the hydrophobic or oleophilic phase, and a polyol phase as described in WO 00/32705.

EXAMPLES

All materials used in the following examples were readily available from standard sources such as Sigma-Aldrich (Belgium) and Acros (Belgium) unless otherwise specified.

Example 1 1. Preparation of the Printing Plate Precursors PPP-00 to PPP-06

Preparation of the Aluminum Support S-01

A 0.3 mm thick aluminum foil was degreased by spraying with an aqueous solution containing 26 g/l NaOH at 65° C. for 2 seconds and rinsed with demineralised water for 1.5 seconds. The foil was then electrochemically grained during 10 seconds using an alternating current in an aqueous solution containing 15 g/l HCl, 15 g/l SO₄ ²⁻ ions and 5 g/l Al³⁺ ions at a temperature of 37° C. and a current density of about 100 A/dm². Afterwards, the aluminum foil was then desmutted by etching with an aqueous solution containing 5.5 g/l of NaOH at 36° C. for 2 seconds and rinsed with demineralised water for 2 seconds. The foil was subsequently subjected to anodic oxidation during 15 seconds in an aqueous solution containing 145 g/l of sulfuric acid at a temperature of 50° C. and a current density of 17 A/dm², then washed with demineralised water for 11 seconds and dried at 120° C. for 5 seconds.

The support thus obtained was characterized by a surface roughness Ra of 0.35-0.4 μm (measured with interferometer NT1100) and had an oxide weight of 3.0 g/m².

Photopolymerisable Layer

The printing plate precursor PPP-00 to PPP-06 were prepared by coating onto the above described support S-01 the components as defined in Table 1 dissolved in a mixture of 35% by volume of MEK and 65% by volume of Dowanol PM (1-methoxy-2-propanol, commercially available from DOW CHEMICAL Company). The coating solution was applied at a wet coating thickness of 30 μm and then dried at 120° C. for 1 minute in a circulation oven. Dry coating weight 1,295 g/m²

TABLE 1 Printing plate precursors PPP-00 to PPP-07 Ingre- PPP- PPP- PPP- PPP- PPP- PPP- PPP- dients 00 01 02 03 04 05 06 mg/m² comp comp inv comp inv comp inv Binder (1) 120.0 120.0 120.0 120.0 120.0 120.0 120.0 Tegoglide 1.5 1.5 1.5 1.5 1.5 1.5 1.5 410 (2) Sartomer 240 240 240 240 240 240 240 CN104 (3) Sartomer 90 90 90 90 90 90 90 CN-UVE 151M (4) Mono 240 240 240 240 240 240 240 Z1620 (5) Disperbyk 45 45 45 45 45 45 45 182 (6) HABI 1-2 3 3 3 3 3 3 3 (7) MBT (8) 21 21 21 21 21 21 21 Sipomer 225 225 225 225 225 225 225 PAM 100 (9) Albritect 30 30 30 30 30 30 30 CP 30 (10) Fluomix 60 — 60 60 60 60 60 (11) Omnicat — 200 200 — — — — 440 (12) Onium-01 — — — 200 — — — (13) Onium-02 — — — 200 — — (14) Onium-03 — — — — 200 — (15) Onium-04 — — — — — 200 (16) 1) Binder represents Alberdingk U180, an aliphatic polyester polyurethane commercially available as a 50 wt. % aqueous dispersion commercially available from Alberdingk Boley 2) Tegoglide 410 is a surfactant commercially available from Evonik Tego Chemie GmbH; 3) CN 104 is an epoxy acrylate oligomer commercially available from Arkema; 4) CN-UVE 151M is an epoxy diacrylate monomer commercially available from Sartomer 5) Mono Z1620 is a solution in MEK containing 30 wt % of a reaction product from 1 mole of 2-hydroxyethylmethacrylate and 0.5 mole of 2-(2-hydroxyethyl-piperidine); 6) Disperbyk 182 is commercially available from BYK Chemie GmbH.; 7) HABI 1-2 is a photoinitiator, commercially available from Hodogaya Chemical; 8) MBT is 2-mercapto-benzimidazool; 9) Sipomer PAM 100 is a methacrylate phosphonic ester commercially available from Rhodia; 10) Albritect CP 30, is a copolymer of vinylphosphonic acid and acrylic acid commercially available as a 20 wt % aqueous dispersion from Rhodia 11) Fluomix is a violet sensitizer mixture consisting of the following compounds:

12) Bis(4-methylphenyl)iodonium hexafluorophosphate; 13) (4-methylphenyl, 4 isobutylphenyl)iodonium hexafluorophosphate; 14) Bis(4-tert-butylphenyl)iodonium tetraphenylborate; 15) Tetrabutylammonium tetraphenylborate; 16) Scarat BI85 commercially available from ABCR GmbH & co; B & S Specialites BV; Cray Valley Prod LTD manufacturer and having the following structure:

Overcoat Layer

On top of the photosensitive layer, a solution in water with the composition as defined in Table 2 was coated (40 μm) on the printing plate precursors, and dried at 120° C. for 2 minutes. The so-formed protective top layer OC-1 has a dry thickness or dry coating weight of 1.35 g/m².

TABLE 2 composition of the overcoat INGREDIENT g OC-01 Mowiol 4-88 (1) 0.817 Mowiol 4-98 (1) 0.493 Ebotec MB-SF (2) 0.002 Advantage S (3) 0.027 Lutensol A8 (4) 0.014 (1) Mowiol 4-88 is a partially hydrolyzed polyvinylalcohol from Kuraray. Mowiol 4-98 is a fully hydrolyzed polyvinylalcohol from Kuraray; (2) Ebotec MB-SF is a biocide commercially available from Bode Chemie Hamburg GmbH; (3) Adavantage S is a dewetting agent commercially available from; ISP; (4) Lutensol A8 is a surface active agent commercially available from BASF.

2. Imaging of the Printing Plate Precursors PPP-00 to PPP-06

Exposure was carried out on a Lücher Expose plate-setter, a UV contact frame with band filters to select the 405 nm region or a 395 nm UV LED firejet 200 of Phoseon, at an energy density of 25 mJ/cm².

3. ΔE Measurement

ΔE was calculated from the L*, a* and b* values of the plate precursor before and after imaging.

Lab measurement executed with a GretagMacBeth SpectroEye reflection spectrophotometer with the settings: D50 (illuminant), 2° (Observer), No filter; commercially available from GretagMacBeth. The total colour difference ΔE is a single value that takes into account the difference between the L*, a* and b* values of the image areas and the non-image areas:

ΔE=√{square root over (ΔL ² +Δa ² +Δb ²)}

The higher the total colour difference ΔE, the better the obtained contrast. The contrast between image and non-image areas results in the occurrence of a print-out image.

4. Results

A solid pattern was imaged on the printing plate precursors PPP-00 to PPP-06 and the L*a*b* values of both the non-image areas and the solid imaged area were measured and the respective delta E (ΔE) values were calculated. Table 3 (Printing plate precursors PPP-00 to PPP-06 with an overcoat layer) and Table 4 (Printing plate precursors PPP-00 to PPP-06 without an overcoat layer) summarise the obtained results.

TABLE 3 Obtained contrast for PPP-00 to PPP-06 with an OC Printing plate precursor PPP-0X Fluomix cation Anion ΔE (1) PPP-00 fluomix — — <0.5 Comp PPP-01 — Iodonium PF⁶⁻ 1.87 Comp PPP-02 fluomix Iodonium PF⁶⁻ 16.07 Inv PPP-03 fluomix Iodonium PF⁶⁻ 13.34 Inv PPP-04 fluomix Iodonium B(Ph)⁴⁻ 4.25 Comp PPP-05 fluomix Ammonium B(Ph)⁴⁻ 1.17 Comp PPP-06 fluomix sulfonium PF⁶⁻ 7.72 Inv (1) A good contrast is defined as ΔE ≥ 5.0.

TABLE 4 Obtained contrast for PPP-00 to PPP-06 without an OC Printing plate precursor PPP-0x sensitizer cation anion ΔE (1) PPP-00 — Iodonium PF⁶⁻ <0.5 Comp PPP-01 fluomix — PF⁶⁻ 0.58 Comp PPP-02 fluomix Iodonium PF⁶⁻ 12.60 Inv PPP-03 fluomix Iodonium PF⁶⁻ 9.65 Inv PPP-04 fluomix Iodonium B(Ph)⁴⁻ 1.79 Comp PPP-05 fluomix Ammonium B(Ph)⁴⁻ 0.31 Comp PPP-06 fluomix sulfonium PF⁶⁻ 8.92 Inv (1) A good contrast is defined as ΔE ≥ 5.0.

The results summarised in Tables 3 and 4 show that at an exposure energy of 25 mJ/cm² a good visual contrast ΔE≥5 is obtained for the inventive printing plate precursors including a coating including the combination of PF₆ ⁻ counter ion as part of an onium structure and fluomix. The onium salt is preferably iodonium salt.

Example 2 1. Preparation of the Printing Plate Precursors PPP-07 to PPP-12

Preparation of the Aluminum Support S-01

See Example 1 above

Photopolymerisable Layer

The printing plate precursor PPP-07 to PPP-12 were prepared by coating onto the above described support S-01 the components as defined in Table 5 dissolved in a mixture of 35% by volume of MEK and 65% by volume of Dowanol PM (1-methoxy-2-propanol, commercially available from DOW CHEMICAL Company). The coating solution was applied at a wet coating thickness of 30 μm and then dried at 120° C. for 1 minute in a circulation oven.

TABLE 5 Printing plate precursors PPP-07 to PPP-12 Printing plate precursor PPP-07 PPP-08 PPP-09 PPP-10 PPP-11 PPP-12 Binder (1) 120.0 120.0 120.0 120.0 120.0 120.0 Tegoglide 410 (2) 1.5 1.5 1.5 1.5 1.5 1.5 Mono Z1620 (3) 240 240 240 240 240 240 HABI 1-2 (4) 3 3 3 3 3 3 MBT (5) 21 21 21 21 21 21 Fluomix (6) 60 60 60 — 60 60 Omnicat 440 (7) — 50 — 50 — — NaPF₆ ⁻ — — 18.5 — 18.5 — Thioxantone (8) — — 60 60 — BF₄ ⁻ — — — — 18.5 (1) To (7): see Table 1 above; (8) Speedcure ITX commercially available from Lambson Limited.

Overcoat Layer

On top of the photosensitive layer, a solution in water with the composition as defined in Table 2 above was coated (40 μm) on the printing plate precursors, and dried at 120° C. for 2 minutes. The so-formed protective top layer OC-1 has a dry thickness or dry coating weight of 1.35 g/m².

2. Imaging of the Printing Plate Precursors PPP-07 to PPP-12

Exposure was carried out on a Lücher Expose plate-setter, a UV contact frame with band filters to select the 405 nm region or a 395 nm UV LED firejet 200 of Phoseon, at an energy density of 25 mJ/cm².

3. ΔE Measurement

ΔE was measured before and after imaging following the method as described in Example 1.

4. Results

A solid pattern was imaged on the printing plate precursors PPP-07 to PPP-12 and the L*a*b* values of both the non-image areas and the solid imaged area were measured and the respective delta E (ΔE) values were calculated. Table 6 summarise the obtained results.

TABLE 6 Obtained contrast for PPP-07 to PPP-12 PPP-0X sensitizer cation anion ΔE (1) PPP-07 fluomix — — 0.19 Comp PPP-08 fluomix iodonium PF⁶⁻ 10.74 Inv PPP-09 fluomix sodium PF⁶⁻ 0.45 comp PPP-10 thioxanthone iodonium PF⁶⁻ 0.16 comp PPP-11 fluomix sodium PF⁶⁻ 0.16 Comp thioxanthone PPP-12 fluomix — BF⁴⁻ 0.48 comp (1) A good contrast is defined as ΔE ≥ 5.0.

The results summarised in Table 6 show that at an exposure energy of 25 mJ/cm² a good visual contrast ΔE≥5 is obtained for the inventive printing plate precursors including a coating including the combination of PF₆ ⁻ counter ion as part of an onium structure and fluomix.

Example 3 1. Preparation of the Printing Plate Precursors PPP-14 to PPP-16

Preparation of the Aluminum Support S-01

See Example 1 above

Photopolymerisable Layer

The printing plate precursor PPP-14 to PPP-16 were prepared by coating onto the above described support S-01 the components as defined in Table 7 dissolved in a mixture of 22% by volume of MeOH and 77% by volume of Dowanol PM (1-methoxy-2-propanol, commercially available from DOW CHEMICAL Company) and 1% by volume monoethylene glycol. The coating solution was applied at a wet coating thickness of 26 μm and then dried at 120° C. for 1 minute in a circulation oven.

TABLE 7 Printing plate precursors PPP-14 to PPP-16 Printing plate precursor PPP-14 PPP-15 PPP-16 Binder (1) 436.0 436.0 436.0 Edaplan LA411 (2) 3.3 3.3 3.3 Phosphoric acid 17.3 17.3 17.3 Diazonium polymer (3) 436 436 436 Fluomix (4) — 50 50 Omnicat 440 (5) — 175 175 (1) PAL 166, Polyvinylbutyral, vinyl alcohol, vinylbutyral copolymer, commercially available form Agfa NV; (2) Edaplan LA411: 10 wt % solution of Edaplan LA 411, a modified siloxane-glycol copolymer commercially available from MUNZING CHEMIE, in Dowanol PM; (3) Diazo resin STE1428 commercially available from Clariant Benelux NV; (4) and (5) see Table 1 above

2. Imaging of the Printing Plate Precursors PPP-14 to PPP-16

Exposure was carried out on a Lücher Expose plate-setter, a UV contact frame with band filters to select the 405 nm region or a 395 nm UV LED firejet 200 of Phoseon, at an energy density of 70 mJ/cm².

3. ΔE Measurement

ΔE was measured before and after imaging following the method as described in Example 1.

4. Results

A solid pattern was imaged on the printing plate precursors PPP-07 to PPP-12 and the L*a*b* values of both the non-image areas and the solid imaged area were measured and the respective delta E (ΔE) values were calculated. Table 8 summarise the obtained results.

TABLE 8 Obtained contrast for PPP-14 to PPP-16 PPP-14 PPP-15 PPP-16 ΔE (1) No color 17.8 15.3 formation (1) A good contrast is defined as ΔE ≥ 5.0.

The results summarised in Table 8 show that a good visual contrast ΔE 5 is obtained for the inventive printing plate precursors including a coating comprising a diazonium salt, an onium structure and fluomix.

Example 4 1. Preparation of the Printing Plate Precursors PPP-17 to PPP-19

Preparation of the Aluminum Support S-01

See Example 1 above

Photopolymerisable Layer

The printing plate precursor PPP-17 to PPP-19 were prepared by coating onto the above described support S-01 the components as defined in Table 9 dissolved in a mixture of 35% by volume of MEK and 65% by volume of Dowanol PM (1-methoxy-2-propanol, commercially available from DOW CHEMICAL Company). The coating solution was applied at a wet coating thickness of 30 μm and then dried at 120° C. for 1 minute in a circulation oven.

TABLE 9 Printing Plate precursors PPP-17 to PPP-19 Ingredients PPP-17 PPP-18 PPP-19 mg/m² inv inv inv Tegoglide 410 (1) 1.5 1.5 1.5 Sartomer CN104 (2) 220.4 220.4 220.4 Sartomer CN-UVE 151M (3) 81.2 81.2 81.2 Mono Z1620 (4) 220.4 220.4 220.4 HABI 1-2 (5) 2.3 2.3 2.3 MBT (6) 18.6 18.6 18.6 Sipomer PAM 100 (7) 208.8 208.8 208.8 Albritect CP 30 (8) 27.3 27.3 27.3 Fluomix (9) 57.9 57.9 57.9 Omnicat 440 (10) 197.2 197.2 197.2 Aerosil 150 (11) 92.8 92.8 92.8 Gohsefimer L5407 (12) 110.2 220.4 — Poval LM30 (13) — — 110.2 (1) to (10) see Table 1 above; (11) Fumed unmodified silica, 20% dispersion in water with Bykjet 9152 (15% aerosil, 5% Bykjet); (12) Polyvinyl alcohol supplied by Nippon Gohsei; (13) Polyvinyl alcohol supplied by Kuraray.

Overcoat Layer

On top of the photosensitive layer of PPP-18 a solution in water with the composition as defined in Table 2 above was coated (40 μm) on the printing plate precursors, and dried at 120° C. for 2 minutes. The so-formed protective top layer OC-1 has a dry thickness or dry coating weight of 1.35 g/m².

2. Imaging of the Printing Plate Precursors PPP-17 to PPP-19

Exposure was carried out on a Lücher Expose plate-setter, a UV contact frame with band filters to select the 405 nm region or a 395 nm UV LED firejet 200 of Phoseon, at an energy density of 25 mJ/cm².

3. Development and Printing

Printing plates PP-17 to PP-19 were evaluated for development on-press.

The printing plates were mounted on a Heidelberg GTO52 dalhgren press using K+E Skinnex 800 SPEED IK black ink (trademark of BASF Druckfarben GmbH) and 4 wt % Prima FS303 SF (trademark of Agfa Graphics) and 8% isopropanol in water as fountain solution. A compressible blanket was used and printing was performed on non-coated offset paper. Prior to paper feeding, 10 press revolution with only the dampening system followed by 5 revolutions with only the inking rollers was performed. Up to 500 sheets were printed and visual assessment of every 10 sheets was performed to evaluate toning (i.e. accepting ink) in the non-image areas.

4. Toning Behaviour

Toning behaviour of the on-press developed printing plates was visually assessed every 10 printed sheets. The results are given in Table 10.

TABLE 10 Toning behaviour of printing plates PP-17 to PP-19 Printing Plate Overcoat * Toning behaviour** PP-17 no 50 inventive PP-18 no 30 inventive PP-18 yes 100 inventive PP-19 no 20 inventive *see Example 1 above; **amount of sheets required to obtain toning-free sheets by visual assessment.

The results in Table 10 indicate that the clean out in terms of toning behaviour of the inventive printing plates PP-17 to PP-19 including iodonium salt and fluomix is good to very good after processing on-press. 

1. A negative-working lithographic printing plate precursor comprising: a coating comprising at least one sensitizer and an onium salt; wherein the sensitizer and the onium salt are capable of inducing a print-out image upon UV light exposure.
 2. The lithographic printing plate precursor according to claim 1 wherein the coating is stable for at least five minutes in office light.
 3. The lithographic printing plate precursor according to claim 1 wherein the sensitizer is represented by Formula II:

wherein R¹ to R¹⁴ independently represent hydrogen, an alkyl group, an alkoxy group, a cyano group or a halogen.
 4. The lithographic printing plate precursor according to claim 3 wherein at least one of R¹ to R¹⁰ represents an alkoxy group having more than 1 carbon atom.
 5. The lithographic printing plate precursor according to claim 1 wherein the sensitizer is a mixture comprising Formulas III, IV, and V:


6. The printing plate precursor according to claim 1 wherein the sensitizer has an absorption maximum in a wavelength range from 320 nm and 500 nm.
 7. The printing plate precursor according to claim 1 wherein the onium salt is an optionally substituted iodonium or sulfonium compound.
 8. The printing plate precursor according to claim 1 wherein the onium salt is an optionally substituted diaryliodonium or triaryl sulfonium compound.
 9. The printing plate precursor according to claim 1 wherein the onium salt has hexafluorophosphate as counter ion.
 10. The printing plate precursor according to claim 1 wherein the onium salt is present in an amount comprised between 1% wt and 25% wt.
 11. The printing plate precursor according to claim 1 wherein the coating further comprises a radical polymerisation initiator, a radically polymerisable compound, and a binder polymer.
 12. The printing plate precursor according to claim 1 wherein the coating further comprises a diazonium compound.
 13. A method for making a printing plate comprising: image-wise exposing the printing plate precursor as defined in claim 1 to UV radiation whereby a lithographic image consisting of image areas and non-image areas is formed and whereby a colour change in the image areas is induced; and developing the exposed precursor.
 14. The method according to claim 13 wherein the colour change is characterized by a CIE 1976 colour distance ΔE between the image and non-image areas of at least
 5. 15. The method according to claim 13 wherein the energy density of the UV radiation is between 10 mJ/cm² and 150 mJ/cm².
 16. A method for making a printing plate comprising: image-wise exposing the printing plate precursor as defined in claim 3 to UV radiation whereby a lithographic image consisting of image areas and non-image areas is formed and whereby a colour change in the image areas is induced; and developing the exposed precursor.
 17. The method according to claim 16 wherein the colour change is characterized by a CIE 1976 colour distance ΔE between the image and non-image areas of at least
 5. 18. The method according to claim 16 wherein the energy density of the UV radiation is between 10 mJ/cm² and 150 mJ/cm².
 19. A method for making a printing plate comprising: image-wise exposing the printing plate precursor as defined in claim 5 to UV radiation whereby a lithographic image consisting of image areas and non-image areas is formed and whereby a colour change in the image areas is induced; and developing the exposed precursor.
 20. The method according to claim 19 wherein the colour change is characterized by a CIE 1976 colour distance ΔE between the image and non-image areas of at least
 5. 